Horizontal directional drilling machines are used to install underground utilities or other objects. This technology is gaining widespread favor because it minimizes ground surface disruption and the likelihood of damaging already-buried objects.
Horizontal directional drilling operations generally consist of using the drilling machine to advance a drill string through the subterranean earth along a preselected path. The path is ordinarily selected so as to avoid already-buried objects such as buried utilities. Certain aspects of the drilling machine and the manner with which it acts on the drill string are included in U.S. Pat. No. 6,085,852 and U.S. Pat. No. 5,799,740, the contents of both are fully incorporated by reference herein.
The drilling machine generally comprises a frame, an anchoring system, a drive assembly mounted to the frame and connectable to the uphole end of the drill string, and a bit connected to the downhole end of the drill string. The drive assembly provides thrust and rotation to the drill string which, in turn, thrusts and rotates the bit through the subterranean earth, forming a borehole. The drive assembly generally comprises one or more power sources for thrusting and rotating the drill string. The drill string is advanced in a substantially straight line direction by a simultaneous rotating and thrusting of the drill string by the drive assembly. To change the direction, conventional steering techniques are used such as those associated with a slant-faced bit. This type of bit is, after being oriented in the desired direction, advanced without drill string rotation to change course of the borehole. When the borehole is completed, typically the bit is replaced with a backreaming tool. Then the drive assembly is used to provide pullback force together with rotation to the drill string which, in turn, will pullback and rotate the backreamer back through the borehole to pack and finally size the borehole.
There are times when it is desirable to maintain the thrust and rotation applied to the drill string for a desired duration. This could be accomplished by manually stroking a control device such as an electric joystick or lever, hydraulic joystick or lever, or manual joystick or lever, to a desired position. Adjustments are made until the drilling machine is operating at a perceived optimum level. The operator then must try and maintain these operational levels by holding the joystick or lever in the desired position, often for long periods of time. When the operator releases the thrust and rotation control for any reason, such as to either add or remove a pipe segment to the drill string, the desired operational levels must be redetermined or located again by trial and error. Additionally, because of human fatigue and uncertainty of actual operational levels, it proves very difficult for an operator to maintain these operational levels for long periods of time.
Some recent useful improvements to controls on directional drilling units have included adding a speed limiting potentiometer on either the thrust or rotation circuits to set a new maximum thrust or rotation level when the joystick or lever is fully stroked. This setup still requires the operator to use the joystick or lever to control rotation or thrust but gives the operator better control of the operational levels in these conditions because the control limit can be set against a stop determined by the limit level set by the potentiometer. Setups of this type on directional drilling units have allowed successful drilling in hard ground conditions and rock where productive manual control previously was nearly impossible or, at best, very tedious. It has also been found to be effective with both conventional bits and mud motors. However, the operator must still hold the control device for extended periods of time. These events and others have led to the development of the s stem of the present invention.